1. Field of the Invention
This invention relates to data processing systems, and more particularly to image compression systems.
2. Description of Related Art
Computer based image compression systems are used for many purposes, including facsimile transmission. FIG. 1 shows a block diagram of an image compression system 20. Scanner 26 scans paper sheet 28 and digitizes the image on the sheet 28. The image data from scanner 26 are transferred through image side system bus 30 to memory 34 ("image" memory).
The image data typically occupy much memory space and take a long time to transmit. They are therefore compressed for long-term storage and transmission. The compression involves coding the image data according to a compression algorithm. Compression/expansion processor ("compressor/expander") 36 reads the image data from image memory 34 through image side system bus 30, compresses the data and writes the compressed data to disk 38 or main memory 40 through host side system bus 42. The central processing unit ("CPU") 44 reads the compressed data from disk 38 or main memory 40 and writes the compressed data through host side system bus 42 to modem 46. Modem 46 transmits the compressed data through communication link 48 (for example, a telephone line). The compressed data are received by a FAX machine or another image compression system on the receiving end of the communication link.
When a FAX machine or another image compression system sends compressed data to image compression system 20, image compression system 20 receives the compressed data through communication link 48 at modem 46. Under the control of CPU 44, the compressed data are transferred through host side system bus 42 to disk 38 or main memory 40. Compressor/expander 36 reads the compressed data from disk 38 or main memory 40 through host side system bus 42, expands the data and writes the expanded (image) data to image memory 34 through image side system bus 30. From image memory 34, the image data are transferred to printer 52 for printing or to video terminal ("CRT") 54 for display. See Peter M. Fuchs, Compressing data conserves memory in bit-mapped displays, EDN. Vol. 31, No. 22.
FIG. 2 shows a block diagram of another image compression system 60. Dual-ported memory 64 replaces image memory 34 of FIG. 1. Dual-ported memory 64 is used to store both image data and compressed data. Dual-ported memory 64 has image side port 66 accessible from image side system bus 30 and host side port 68 accessible from host side system bus 42. Image compression system 60 offers greater flexibility in accessing data for it allows accessing both image and compressed data from both image side system bus 30 and host side system bus 42. See Govind Kamath, PC Facsimile Transfer Steps Up, Electronic Design, Mar. 9, 1989, pages 59-66.